In order to gain a more physical insight,
we compare the limiting case of our exact analytical expressions presented in Eq. REF , when \(\theta =0\)  we recover the well known results for the radiation pressure acting on an infinite slab with gain/loss due to a normal incidence [1]}, [2]}, [3]}, [4]}.
\(\mathbf {F}{{(0)}}= 2\varepsilon E_{0}^2\biggl [1+\vert r_{TE} \vert ^2- \vert t_{TE} \vert ^2 \biggl ]\mathbf {\hat{z}}. \) 
